Multilevel converters, especially inverters have been used for power conversion in high-power applications such as medium voltage grid (2.3 KV, 3.3 KV, or 6.9 KV) to reduce the switch voltage stress, and photovoltaic (PV) applications to reduce the filter size. Compared to two-level voltage source converters, the advantages of multilevel converters are lower voltage stress, higher efficiency, smaller filter size and lower common-mode voltage.
There are three types of traditional five-level topologies: the neutral-point-clamped (NPC) type shown in FIG. 1(a), flying-capacitor (FC) type shown in FIG. 1(b), and cascaded H-bridge (CHB) type shown in FIG. 1(c). It can be seen from the FIG. 1, there are eight controllable switches in each type of traditional topologies. The number of controllable switches is so big that it increases the cost. For the five-level NPC topologies, they generate more voltage levels from the neutral point voltage by adopting diodes. The drawback is the increasing number of switching devices and diodes when the number of voltage level increases. The FC type outputs voltage levels by using the flying-capacitor as part of power supply. However, more voltage levels need more flying-capacitors and more complexity of control strategy to balance the voltage of each flying-capacitor. The CHB multilevel converters use series-connected H-bridge cells with an isolated DC voltage source connected to each cell. Similarly, to output more voltage levels, more cells are needed. This will lead to impracticality of this type of topology since more DC sources are required.
Active-neutral-point-clamped (ANPC) converter is of multilevel topology. It combines the features of NPC and FC topology. The ANPC topology is attracting more and more attention nowadays because of high efficiency and multi-level output. Eight controllable semiconductor switches are used in the present five-level ANPC topology. That is to say, the number of controllable semiconductor switches is big. So they have high cost.